1. Field of the Invention
The present invention relates to a power transmission apparatus for vehicles which transmits a driving force from one transmission shaft to the other transmission shaft by hydraulic pressure generated in a hydraulic pump, more particularly, it relates to a power transmission apparatus for vehicles which, in a four-wheel drive vehicle, transmits the driving force from the transmission shaft on the front wheel side to that on the rear wheel side or vice versa to realize a four-wheel driving state.
2. Description of the Related Art
In recent years, a four-wheel drive vehicle is particularly highlighted as the vehicle which can realize a comfortable driving regardless of the road conditions, natural conditions such as weather and driving conditions, because of its high running stability in specific surface conditions such as the snow and gravel roads and its superb driving stability during the high speed driving and at acceleration and deceleration also on the ordinary roads.
Furthermore, recently, a so-called full-time four-wheel drive vehicle constructed always to obtain substantially the four-wheel driving state by having a power transmission apparatus, which changes power distributions to the front and rear wheels responsive to the rotational speed difference generated therebetween when it took place, is the mainstream.
As such a four-wheel drive vehicle, it is the general trend to provide the power transmission apparatus which distributes the driving force responsive to the rotational speed difference generated between the front and rear wheels, in the midway of a transmission shaft between the front and rear wheels. As such a power transmission apparatus, there is the one which utilizes hydraulic pressure generated in a hydraulic pump, particularly, a vane pump which can be easily made to be small sized and lightweighted and having durability.
The vane pump, as already known, is provided with a casing constituted by a cam ring formed annually with deviation in thickness and side plates mounted on its sides, and a short cylindrical rotor, onto which a plurality of plate vanes are disposed substantially in equal intervals in a circumferential direction, mounted movable in a radial direction, and is constructed to form a pump chamber surrounded by the outer surface of the rotor, the inner surface of the cam ring, and further, by these and side walls of the side plates, by containing the rotor rotatably coaxially in an inner space of the casing. By sealing working oil introduced into the pump chamber between the vanes adjoining each other, pressure is raised responsive to the rotation of the rotor.
The power transmission apparatus is so designed that, by securing the rotor coaxially to the driving shaft connected to either of the front and rear wheels, and securing the casing coaxially to the driving shaft connected to the other wheels, relative rotation corresponding to the rotational speed difference between the front and rear wheels is generated between the rotor and the casing. At this time, since the hydraulic pressure generated in the vane pump chamber corresponding to the relative rotation or the rotational speed difference between the front and rear wheels, acts to restrain the relative rotation between the rotor and the casing, the driving force responsive to the rotational speed difference between the front and rear wheels is transmitted from one of the front and rear wheels to the other via the hydraulic pressure, thereby a desired four-wheel driving state is realized.
Now, in the four-wheel drive vehicle, for example, in case of driving on bad roads such as the snow and gravel roads, it is desirable to obtain a rigid coupling state as much as possible to secure the stable running. Meanwhile, in case of driving on the paved road, it is desirable to obtain a relatively loose coupling state to change transfer characteristics of the driving force between the front and rear wheels so as to prevent a tight corner braking phenomenon reliably.
Besides, it is also desirable to change the transfer characteristics not only responsive to the road conditions aforementioned, but also to various running states such as the vehicle speed, steering angle and braking.
Moreover, in the power transmission apparatus using a hydraulic pump, the power transmission between the front and rear wheels is effected via the hydraulic pressure generated in the hydraulic pump, thus the magnitude of the transmission force corresponds to the hydraulic pressure. However, as the power transmission apparatus (casing of the vane pump) rotates by itself, working oil in the hydraulic pump is sealed inside an oil tank which is formed as surrounding around the casing and restricted in size, and recirculated for use. Therefore, the temperature rise of the working oil during operation is unavoidable, results in pressure drop in the hydraulic pump due to the lowering of viscosity in connection with the temperature rise, whereby the transfer characteristics of the driving force between the front and rear wheels may change to cause insufficient transmission of the driving force corresponding to the rotational speed difference between the front and rear wheels.
In view of such circumstances, the inventors have proposed, in the U.S. Pat. No. 4,850,447, a power transmission apparatus, in which changes in transfer characteristics of the driving force corresponding to the running state, and corrections of transfer characteristics change in connection with the change in oil temperature are made possible.
Frankly speaking, the invention is constituted such that, by providing a variable throttle driven electrically or hydraulically in a n oil discharge passage of the hydraulic pump, and changing an opening of the variable throttle to change passage resistance in the oil discharge passage, the pressure characteristics of the hydraulic pump can be changed. On the other hand, sensors for detecting various state quantities relating to the running states such as the vehicle speed, steering angle and accelerator opening are arranged on the vehicle body. In this power transmission apparatus, an appropriate transfer characteristics responsive to the running states is obtained by adjusting the energizing current to the solenoid, and changing the throttle opening of the variable throttle continuously in response to the detect on results of respective sensors.
In such power transmission apparatus for vehicles, the transfer characteristics change in connection with the oil temperature rise may be corrected by changing the throttle opening responsive also to the detection result of working oil temperature, thereby desired transfer characteristics can always be obtained.
In the variable throttle adopted in the aforesaid U.S. Pat. No. 4,850,477, however, in case of electrical drive, wirings to a drive source are required, and in case of hydraulic drive, hydraulic pipings to the drive source are required respectively. However, since it is difficult to fix the wirings or pipings to the power transmission apparatus which is rotating as aforementioned, they are fixed to the stationary portion in the vicinity of the apparatus. Accordingly, the oil discharge passage of the hydraulic pump aforementioned is inevitably so constructed in a complicate way that, it extends from the rotating power transmission apparatus to the variable throttle provided on the stationary portions, and from which it returns further to the power transmission apparatus.
Therefore, in the U.S. Pat. No. 4,850,447, oil discharged from the hydraulic pump is temporarily once introduced into the power transmission apparatus from the oil passage formed in the axis of the rotary shaft of the rotor, led to the stationary portion in the vicinity of the supporting position of the rotary shaft outside the power transmission apparatus, and after passing through the variable throttle fixed thereon, guided into the oil passage formed at a portion of the casing near the supporting position thereof, and returned to the oil tank.
In the conventional power transmission apparatus in which the transfer characteristics of the driving force can be changed, oil discharge passages having a complicated construction is necessary, accordingly, on manufacturing the apparatus, not only a great deal of works are required for forming the oil passages, but also a troublesome work of matching the oil passages here and there one another is encountered at assembling.
Also, in addition to a complicated construction of the driving solenoid for changing the opening of the variable throttle continuously, a control unit for controlling the energizing current to the solenoid becomes complicated, results not only in a high cost but also a high provability of defective operation of the solenoid and control unit as well as a low reliability are feared.
Furthermore, in order to meet the requirements stated above, a variable throttle whose opening is adjustable from the outside must be provided in the middle of the oil discharge passage formed in the rotating casing, results in a difficult arrangement.
That is, generally, it may be considered to construct in such a manner that, a portion connecting the casing of the vane pump and a transmission shaft corresponding thereto is made cylindrically so as to incorporate a variable throttle and a magnetic member connected to one side of the variable throttle slidably in an axial direction on the axis thereof, and at the same time, an exciting coil fixed to any position is provided around the cylindrical portion to move the magnetic member, thereby adjusting the opening of the variable throttle.
However, in the case where such a configuration is adopted, in order to focus the magnetic flux generated by energizing the exciting coil on the position where the magnetic member is disposed to move the magnetic member reliably, it is important to cut off the magnetic flux leakage into the cylindrical portion. In order to obtain this end, a ring made of non-magnetic member is welded on the middle of the cylindrical portion to cut off the magnetic flux leakage of the magnetic member, thereby the magnetic flux is focused on the magnetic member. However, in the case where the magnetic member and the non-magnetic member are welded therebetween, a sufficient welding strength can not be guaranteed, so that when a large transmission torque is exerted on the cylindrical portion, the welded portion may break.
Against the fact that the exciting coil which is fixed to any portion of the vehicle body, the cylindrical portion containing the magnetic member is rotated together with the power transmission apparatus, therefore, variations of the relative position in a radial direction of the magnetic member against the exciting coil due to vibration of the vehicle body during running can not be avoided. Therefore, there is the possibility that the intensity of magnetic field in the position where the magnetic member is disposed becomes unstable to cause fluctuations of the opening of the variable throttle, thereby the desired transfer characteristics can not be obtained and further, in the case where vibration of the vehicle body is large, the exciting coil may be damaged by collision with the cylindrical portion.
Moreover, there are some difficulties in that, the driving solenoid of the variable throttle becomes complicated in construction, and in order to obtain the appropriate throttle opening for respective detection results of plural kinds of state quantities and to realize the opening, an opening control unit for adjusting the energizing current to the solenoid continuously becomes complicated in construction, thus in addition to the high manufacturing cost, results in a high probability of defective operation of the solenoid and opening control unit and a low reliability.